Current United States emission standards for heavy-duty vehicles powered by diesel engines require all new engines not to emit more than 4.0 g/bhp-hr (grams per brake horsepower--hour) of NOx (Nitrogen oxides). Engine manufacturers were able to meet this standard via various improvements to the engine design, advancement to the fuel injection equipment, sophisticated engine controls, etc. Tightening emission regulations by the United States Environmental Protection Agency (EPA) will soon require the heavy-duty diesels to produce 2.5 g/bhp-hr or less NOx and particulate matter emissions of 0.10 g/bhp-hr or less by the year 2002. Meeting this new standard will most likely require use of an EGR system.
For almost two decades, EGR systems have been known to reduce NOx emissions and, as now developed, have been successfully applied to modem gasoline engines to meet past and current emission regulations. Because of the tightening NOx standards for diesel engines, EGR systems have been and are currently being investigated for application to diesel engine emission systems for reduction of NOx. However, application of EGR systems to diesel engines present several distinct challenges or problems unique to diesel engines which include the following:
To overcome the insufficient differential pressure problem set forth in item (A), a so-called high pressure EGR (also known as "short-route") system has been developed. This system is schematically diagramed in prior art FIGS. 1 and 1A. FIG. 1 shows an engine 1 equipped with a turbocharger 2. Ambient air is drawn into the engine through an air filter 3 where it is compressed through a compressor or charger 4 of turbocharger 2 and subsequently cooled through an intercooler 5 before entering into engine 1. Exhaust gases from engine 1 pass through the turbine 6 (of turbocharger 2 which drives compressor 4) before passing through an exhaust system to atmosphere. A high pressure EGR loop, shown as dashed line 7, re-circulates a slip stream of exhaust gases between an inlet end 7i of EGR loop 7 that is upstream of turbine 6 and an exit end 7ii of EGR loop 7 that is downstream of boost air intercooler 5. A small differential pressure naturally exists between inlet and exit ends 7i, 7ii of the EGR loop. The differential pressure in EGR loop 7 is artificially enhanced by de-rating turbocharger 2. On a conventional turbocharger this is achieved through an appropriate internal geometry affecting exhaust and/or airflow conditions. On a modern variable geometry turbocharger (VGT) this is achieved through a control regulated by the engine control unit (ECU). An EGR valve 10 controls EGR flow and is typically vacuum or pressure operated, but can also be controlled through the engine's ECU. As described, the high pressure EGR system of FIG. 1 will provide sufficient differential pressure through EGR line 7 for exhaust gas flow although de-rating the turbo charger reduces its efficiency. Further, engine contamination is limited to internal components only. That is, because the EGR exit 7ii is downstream of the intercooler 5, contamination resulting from the exhaust gases in EGR loop 7 is not present in compressor 4 and intercooler 5.
However, a high pressure EGR system does not eliminate the contamination problem of items B and C above. In fact, engines equipped with a high-pressure EGR system suffer durability problems caused by the dirty exhaust in EGR line 7 being re-circulated into the engine. Further, the presence of the exhaust in the engine's combustion chamber and the artificially de-rated turbocharger reduce the engine's fuel efficiency.
In addition, there are more subtle problems caused by a high pressure EGR system, which have significant impact on the engine. By positioning the loop inlet (pick-up) 7i upstream of turbine 6, the temperature of the EGR gas is higher than what it might otherwise be, and because of the short travel distance to the intercooler and the engine combustion chamber a mixer, usually in the form of a venturi at loop outlet (return) 7ii is required. Because of high EGR temperature an additional cooler 8 in the EGR line 7 is required. Such an EGR cooler typically utilizes coolant liquid from the engine cooling system and imposes additional load on the system. Alternatively, if an EGR cooler is simply not applied the inlet gas temperature into the engine increases, causing an additional fuel penalty. Significantly, EGR cooler 8 has proven to be an expensive and non-durable component. It is typically a gas-to-water heat exchanger that plugs up quickly due to contamination with particulate matter. It is believed that contamination of the EGR cooler 8 is being addressed by others who are experimenting with inserting a catalyst or a soot filter 9 in EGR line 7. The belief is that a catalyst/soot filter will clean the EGR gas to avoid plugging or contamination of EGR cooler 8. However, the successful implementation of such system has not yet been demonstrated. The addition of a catalyst/soot filter into the high-pressure EGR loop will add additional restrictions to the flow of the EGR gas and may necessitate further de-rating of the turbo-charger causing an additional associated fuel penalty. It is to be recognized that in order to achieve maximum NOx removal effective cooling of the EGR gas is required. However, if the EGR gas is cooled below its dew point to achieve maximum NOx removal, moisture will condense in the EGR loop. This moisture reacts with nitrogen oxides and sulfates forming nitric and sulfuric acids which have a detrimental effect on the metal components of the engine when re-circulated back to the engine's combustion chamber (along with any condensed water which also has a detrimental effect on the engine and the combustion process). This problem is addressed in current EGR systems by maintaining the EGR gas temperature above its dew point. However, this corrective or avoidance action causes under-utilization of the NOx removal capabilities of the EGR system.
Finally, FIG. 1 illustrates that a substantial drawback of the high-pressure EGR system shown is the presence of a relatively complex control system. The basic control for the shown high-pressure EGR loop includes a vacuum or pressure operated EGR flow valve 10, and a more sophisticated system would also control turbocharger 2 through the engine's ECU. In order to assure proper air/gas mixing at the EGR loop outlet 7ii, a venturi or other mixing device is required. The overall complexity of the high-pressure EGR system makes it virtually impractical for retrofit applications. Most present day heavy-duty diesel powered vehicles are not equipped with EGR systems and their ECUs are not coded to control engines equipped with an EGR system. A need exists to equip such vehicles with a passive emission control system that will satisfy emission regulations without substantial modifications to the engine such as rebuilding or replacing the turbocharger, re-coding or replacing the ECU, etc.
An alternative to the high-pressure system is a low pressure EGR (also known as a "long route") system and this invention relates to such a system. A low pressure EGR system re-circulates exhaust gas between the two low pressure points of an engine. Specifically, the EGR line inlet (pick-up) is downstream of the turbine and the EGR line outlet (return) is upstream of the compressor and downstream of the air filter as shown by dot-dash line 7A in FIG. 1A. Until this invention, such a system has not been considered practical for diesel engines because of the following problems:
Specifically, the differential pressure between exhaust and intake of a low-pressure EGR loop is negligible at low and medium loads of the engine. Exhaust gas re-circulation occurs at high engine loads only. It is known, that throttling the tailpipe to create additional backpressure in the exhaust or throttling the intake airflow to create vacuum in the intake can artificially increase the EGR rate. However, such modifications produce limitations which adversely affect engine power output, fuel efficiency, durability and safety considerations that do not allow for a required EGR flow rate during high load engine operations. Generally, the principal reasons, until this invention, for low-pressure EGR systems not being accepted by the industry are:
There are, however, several attractive features of low-pressure EGR systems such as naturally lower EGR temperatures when compared to the high-pressure EGR system.
It should also be mentioned that a hybrid EGR, a combination of high and low-pressure EGR systems, also exists. Such a system would re-circulate exhaust between an EGR inlet upstream of the turbo and EGR outlet upstream of the compressor and downstream of the intake air filter as shown by the dot-dot-dash line 7B in FIG. 1A. The attractiveness of such a hybrid system resides in its ability to address the problem of insufficient differential pressures in the EGR loop, i.e., problem item A. That is, a hybrid high/low pressure EGR system will allow for relatively large EGR flow rates without artificially creating effects to induce differential pressures in the EGR line, such as de-rating the turbocharger. However, contamination problems B and C must be resolved as well as engine performance and durability, complexity of the design and the control(s) for such a system. To be accepted by the industry, such a system requires a particulate free EGR stream along with a very effective EGR cooler.